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My name is Mahesh. I'm a professor of radiology and radiological science from Johns

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Hopkins University School of Medicine, and I'm going to speak on radiation biology.

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Radiation biology is an important topic to be understood by anyone using radiation,

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especially radiology residents and radiologists, and so forth.

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Before I talk into more details about radiation biology, I just want to give an

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overview of biological effect of radiation.

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The biological effect of radiation can be grouped into two categories.

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One is called the stochastic effect, also called as delayed effects.

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The second one is called non-stochastic effect or deterministic or tissue effects.

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The stochastic effects are more examples of cancers in exposed individuals

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observed over three to 20 years after exposure, mutations in offsprings of exposed

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individuals observed in future generation.

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Non-stochastic effect are also called as deterministic or tissue effect.

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They usually occur within short time after the exposure because the direct damage

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to tissue due to local cell death can cause this effect, and these are observable

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within days to weeks. The reason I'm giving this bigger picture is before dwelling

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into more detail, this is really important for our understanding of radiation

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biology, the broader category, at least with the radiation dose levels we use in

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imaging and so forth.

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Here is a cartoonish type of diagram describing the physical and biological

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responses to ionizing radiation.

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Irrespective of the radiation dose level or the dose magnitude, any

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radiation interacting with the body in the human matter, usually it interacts with

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the water molecule because we have a lot more water molecules in the body than any

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other molecules. These interactions can cause what is called as a indirect

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inaction, or it can directly interact with the DNA, causing molecular

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damage, and that we call it a direct action.

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I'm going to go into more details as we go along, but I'm going to give a general

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picture. This molecular damage can either cause mutations or it can repair and we

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don't see any impact. If the mutation happen, then there is a biological response,

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and it can be both genetic, somatic, teratogenic, and death.

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This is the bigger picture of the physical and biological response.

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Also want to draw your attention is to the timescale of this interaction, of any

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radiation interaction. The left hand part takes around 10 to the power of minus 17

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to 10 to the power of minus five second, and a minuscule amount of time.

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On the right-hand side, the repair and the biological response and so forth can

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occur between minutes to decades.

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When I say minutes to decades, that's where the biological response of effects of

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radiation grouped into stochastic effect and tissue effect.